A common type of earth boring bit has rotatable cones. Each cone is rotatably mounted to a bearing pin that depends from the body of the bit. The cones have cutting elements that disintegrate the earth formation as the bit body is rotated. The cutting elements may comprise tungsten carbide compacts press-fitted into holes in the supporting metal of the cones. Alternately, the cutting elements may be milled teeth that are integrally formed from the cone metal.
Each cone has a gage surface that engages the side of the bore hole as the bit rotates. In milled teeth cones, the teeth of the heel row, which is the row closest to the borehole wall, have gage sides that typically blend into the gage surface. The gage surface is an annular area that extends from a backface of the cone and joins the gage sides of the heel row teeth. Often trimmer cutting elements will be located between the heel row primary cutting elements at the outer periphery of the gage surface.
For many years, manufacturers have applied hardfacing to the milled teeth to reduce wear. Typically, the hardfacing is applied to the entire tooth, including its gage side, nose side, leading flank and trailing flank. In milled teeth bits, the trimmer cutting elements may be formed of a hardfacing deposit.
Under very abrasive formation conditions, the gage surface will round over and wear away the hardfacing. The underlying steel areas of the gage surface become exposed. Once exposed, the gage surface has very little resistance to wear under abrasive conditions. The gage surface will quickly wear, and the useful life of the bit will then be over because the bit will no longer be able to drill in-gage, causing bearing failure.
To reduce wear to the gage surface of milled teeth bits, manufacturers have extended the hardfacing from the gage sides of the heel row teeth to the inner edge of the gage surface. These hardfacing deposits were spaced apart from each other around the gage surface. The spaces on the gage surface between the hardfacing deposits were recessed and free of hardfacing.